This invention relates generally to a control circuit for a delayed sweep oscilloscope, and more particularly to a circuit for selecting trigger source, coupling, trigger level, and sweep mode of a delayed sweep oscilloscope.
A block diagram of a conventional delayed sweep oscilloscope is shown in FIG. 1 for better understanding of this invention. Applied to input terminal 10 is an input signal to be observed which is ultimately applied to the vertical deflection plates of cathode-ray tube (CRT) 20 through controllable attenuator (ATT) 12, preamplifier 14, delay line circuit 16 and vertical output amplifier 18. Trigger source circuit 22 is used to select trigger signals from various sources including internal (INT), external (EXT) and power line (LINE) trigger signal sources. The internal trigger signal is derived from vertical preamplifier 14. Although only one vertical channel is shown in this example, most laboratory oscilloscopes include two or more vertical channels. For multi-channel oscilloscopes, internal trigger signals are derived from each channel. The external trigger signal is applied to A EXT connector 24. The LINE trigger signal is coupled to LINE terminal 26 for displaying signals from any power line frequency related circuits. The selected trigger signal from trigger source circuit 22 is applied to trigger coupling circuit 28 for selecting either AC, DC, low frequency reject (LF REJ), or high frequency reject (HG REJ) modes. The trigger signal passed through coupling circuit 28 is then applied to trigger generator 30 for regenerating a fast trigger pulse by comparing the level of such trigger signal with a controllable trigger level from A TRIG LEVEL potentiometer 32. The output trigger pulse from trigger generator 30 is used to initiate A sweep generator 34. A sweep signal from A sweep generator 34 is applied to the horizontal deflection plates of CRT 20 by way of sweep mode selection switch 50 and horizontal output amplifier 52.
B sweep circuitry is essentially the same as the aforementioned A sweep circuitry including trigger source circuit 36, B EXT connector 37, trigger coupling circuit 38, trigger generator 40, B trigger level control potentiometer 42, and B sweep generator 48. The B sweep signal from B sweep generator 48 is also applied to the horizontal deflection plates of CRT 20 by way of switch 50 and horizontal output amplifier 52. Additionally, delay comparator 44 is utilized to compare the A sweep signal with a delay time control potentiometer or delay time position (DTP) potentiometer 46. The comparator output is applied to trigger circuit 40. Although not shown in FIG. 1, trigger circuits 30 and 40 are so designed to operate in either automatic level (AUTO LVL), automatic (AUTO), normal (NORM), or single (SINGLE) mode. The AUTO LVL is a trigger mode to trigger on any desired percentage of the waveform regardless of the DC level and amplitude of the trigger signal as disclosed in U.S. Pat. No. 3,879,669 to Moriyasu. For this purpose, a peak-to-peak trigger signal detector is employed to set both extreme voltages of the trigger level control potentiometer. The AUTO mode is used to free run the sweep generator when the trigger generator is not properly generating trigger pulses at a certain repetition rate higher than a predetermined frequency, e.g. 50 Hz. One example of the AUTO mode is disclosed in U.S. Pat. No. 3,215,948 to Oliver Dalton and includes a trigger pulse interval measurement circuit. The NORM mode is used to initiate the sweep generator only when the trigger signal level crosses the trigger level set by TRIG LEVEL potentiometer 42. No waveform or trace is displayed if not triggered. The B sweep generator has two modes that are different from the A sweep generator. One is run after delay (RUN) and the other is triggerable after delay time (TRIG). In the RUN mode, B sweep generator 48 is only armed by the delay trigger pulse but is triggered (or initiated) on receiving the next subsequent trigger signal from trigger coupling circuit 38.
Sweep mode selection 50 is used to select either the normal sweep from A sweep generator 34 or the delayed sweep signal from B sweep generator 48 to be amplified by horizontal output amplifier 52 for driving the horizontal deflection plates. Sweep gate pulses from A and B sweep generators 34 and 48 are combined together in Z-axis amplifier 54 to control the control grid bias of CRT 20. Sweep mode switch 50 selects the normal sweep signal for observing the entire waveform including the interesting portion to be magnified by the delayed sweep. When selecting the delayed sweep by sweep mode switch 50, only the interesting portion is displayed on the magnified scale.
Control switches for trigger source and coupling selection purposes may be a level control type rotary switch including a control level and a plurality of switch contacts provided directly on an electrical circuit board and a programmable switch actuator cam member.
A disadvantage of the conventional control circuit for a delay sweep oscilloscope is the need for many controls and switches in addition to their indicators to be mounted for both A and B sweep circuits on the front panel despite essentially identical purposes for both sweep circuits. This problem becomes increasingly serious as the number of signal channels increases, such as, for example, a four channel oscilloscope. In addition, these controls and switches for the B sweep circuit are less frequently used as compared with those for the A sweep circuit. It is also very hard to obtain more than five different selections by using a conventional lever switch.
The subject matter of this invention is the use of a single up/down control switch for sequentially or step-by-step selection of a plurality of different modes by continuously or intermittently operating the switch and a common indicator which is preferably made of a plastic overlay on the front panel including a plurality of light sources, e.g. LEDs (light emitting diodes) behind the plastic overlay. Such selection switches and indicators are commonly used for both A and B sweep circuits. Additionally, a single trigger control potentiometer is used for both sweep circuits. Normally, the selection switches, indicators and trigger level control are used for controlling or selecting the switches or potentiometer setting of the A sweep circuit but they may be used for the same purposes of the B sweep circuit by using a shift switch.
It is therefore an object of this invention to provide a simplified control circuit for a delayed sweep oscilloscope.
It is another object of this invention to provide an improved control and indicator apparatus for a delayed sweep oscilloscope.
It is yet another object of this invention to provide a control circuit for a delayed sweep oscilloscope wherein only a single control and indicator are normally used for the A sweep circuit and may also be used for the B sweep circuit as necessary without causing operational inconvenience.
These and other objects as well as construction and advantages will become apparent to those skilled in the art upon reading the following description of the invention when taken in conjunction with the accompanying drawings.